Thermal energy storage compositions used for cool storage have achieved definite success in the marketplace at the present time. Those compositions are presently being used to store coolness during off-peak periods when, because of a differential in consumption rates and/or demand charges, the cost of electrical energy is less, and then release such stored coolness during peak periods when electric utilities must meet greater demands for their services During peak usage periods utilities normally will impose a demand charge based on the maximum number of kilowatts demanded for use during the period, as well as increased rates for electricity, usually measured as kilowatt hours, consumed during the period. It thus has become commercially expedient for a wide variety of users of electricity during peak periods, e.g., industrial plants, hospitals, schools, houses of worship, and the like, to utilize cool storage compositions whereby chillers to chill water to be circulated throughout the buildings are utilized primarily during off-peaks hours.
In a typical cool storage use, a relatively large tank, which may be located below grade, is filled with bottles of a cool storage composition based on sodium sulfate decahydrate. Such bottles are preferably of the utilitarian shape disclosed and claimed in my U.S. patent application Ser. No. 696,529, filed Jan. 30, 1985, and entitled, Nestable, Stackable Containers. During the least expensive off-peak time of use, e.g., midnight to 6 a.m., a chilled fluid such as water is circulated within the tank containing the bottles of salts. That water, which has been chilled to a temperature below about 47.degree. F., is circulated about the stacked, spaced bottles in the tank until the contents of the bottles, which freeze at about 47.degree. F., have been frozen. The repetitive cycle comprises chilling the water, passing it through the tank in contact with the bottles, and then recycling the water to the chiller and the tank again.
After the salts in their bottles have all been frozen, the chillers are deactivated. Then, during peak periods, e.g., noon to 6 p.m. the following day, rather than activate the chiller that consumes large quantities of electricity and results in a high demand charge during the peak period, a pump in the system is used to force line water, normally at a temperature above 47.degree. F., through the tank where that water is chilled to about 47.degree. F. as the frozen salts in their containers begin to melt, thus, acquiring heat from the surrounding water. That chilled water is then pumped through the building to enable the building to be air-conditioned, and then down into the tank, where the water is again chilled to approximately 47.degree. F. In this manner cool storage compositions have been in commercial use to transfer usage of electricity from peak to off-peak periods.
The above described system is what is known as a full storage system; in certain circumstances, according to utility rates, it may be more economic to utilize a partial storage system in which only a part of the cooling needs of the building during peak periods are met by the stored coolness, the other portion being supplied by chilled water directly from the chiller.
As pointed out in my above-mentioned application Ser. No. 126,505, filed Nov. 30, 1987, PCM's presently used commercially as cool storage compositions of matter are those based on Glauber's salt, sodium sulfate decahydrate. The Glauber's salt is a basic part of a fairly sophisticated composition, that cost of which is a definite factor in the achievement of economic viability of cool storage compositions. A typical consumer for such a cool storage composition system will measure the cost effectiveness of the system on the payback period, measured as the amount he has to pay for the cool storage system divided by the annual savings from paying a lower demand charge and the differential between peak and off-peak energy consumption rates.
Although the use of off-peak cool storage systems results in great savings to the environment because less burning of fossil fuels is required, since most electric utilities do not utilize their full, off-peak, base load requirement that must be maintained in any case, the customer is primarily concerned with the number of years that it will take until his out-of-pocket savings balances his expenditure for the system. Generally, a three-year payback period is considered the dividing line so far as attractiveness of the system to the customer is concerned.
As a consequence, it is exceptionally important for commercial viability of a cool storage composition of matter that the composition not only be relatively cheap to manufacture and composed of ingredients that are in plentiful supply and reasonably priced, but of a cool storage composition that will store relatively large quantities of coolness with a given weight of composition. Because, as previously referenced, the cool storage compositions presently based on Glauber's salt are contained within bottles, which are stacked in tanks usually formed from concrete, the greater the cool storage capacity of the composition per unit of weight, the fewer bottles thereof need be utilized in order to store a specific number of BTU's on freezing. Fewer bottles result in a reduction in the plastic from which the bottles are made, and the fewer the number of bottles, the smaller the tank that need be utilized to hold the bottles. Thus, even with comparable costs of materials between Glauber's salt-based storage compositions and other cool storage compositions, great savings can be effected by use of a composition which has a significantly greater heat of fusion, that is, the ability to store more BTU's per pound of material utilized.
While in prior patent applications of which the subject application is a continuation-in-part I have disclosed the use of a sodium hydroxide-water solution which has a freezing/melting point plateau of approximately 41.degree. F., and while the attainment of cool storage compositions having that lower freezing point is believed to be a significant achievement in this art, nevertheless, particularly in retrofit applications where the existing chiller cannot chill water to temperatures below 38.degree. F. for extended periods of time without undue wear, 47.degree. F. water is still a reasonable, low temperature for chilled water. Although the sodium hydroxide/water cool storage compositions of my prior applications are particularly useful when a chiller can easily provide water temperatures as low as 34.degree. F. to 36.degree. F., where a chiller already in place can only consistently produce water at, say 42.degree. F. to 44.degree. F., a 41.degree. F. melting/freezing point plateau composition of matter will be inutile.
Thus, while my prior NaOH/H.sub.2 O compositions that have a freezing/melting point plateau of about 41.degree. F. have been a bellwether for low-temperature coolness storage compositions, those compositions do not have universal applicability. In many applications the demands of the marketplace require a cool storage composition having a melting/freezing point plateau of about 47.degree. F., and sodium sulfate-based compositions are the only ones presently available. However, a 47.degree. F. cool storage composition having improved cool storage capacity, i.e., a higher heat of fusion than Glauber's salt-based compositions, should have a definite place in the market, particularly when that composition has an increased heat of fusion as compared with that of the present, Glauber's salt-based, cool storage compositions.
It is, therefore, a primary object of the present invention to provide a phase change material that will freeze and melt at a plateau at about 47.degree. F., but which will have a higher heat of fusion than conventional, Glauber's salt-based compositions.
It is a further object of my invention to provide a cool storage composition of matter which has an improved heat of fusion on the order of the improvement achieved by my 41.degree. F. melting/freezing point NaOH/water-based compositions, but which will have a melting/freezing point plateau at approximately 47.degree. F. rather than at about 41.degree. F. The present invention is believed to achieve the above objects.